E420 Oil Quantity Indicator

[emerydc8]

For no apparent reason, after my engine comes up to operating temperature (usually after about 5 minutes), the oil quantity light illuminates. When I turn the engine off and restart it, the light appears immediately and does not extinguish for the remainder of the operating cycle.

When the engine has completely cooled down, I can start it up again and the light won't illuminate until it gets back to operating temperature.

The oil quantity is fine--right at the top plastic mark where it's always been.

Any thoughts?